xref: /libcore/ojluni/src/main/java/java/lang/Runtime.java

/*
 * Copyright (C) 2014 The Android Open Source Project
 * Copyright (c) 1995, 2006, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.lang;

import java.io.*;
import java.util.StringTokenizer;
import sun.reflect.CallerSensitive;
import java.lang.ref.FinalizerReference;
import java.util.ArrayList;
import java.util.List;
import dalvik.system.BaseDexClassLoader;
import dalvik.system.VMDebug;
import dalvik.system.VMStack;
import dalvik.system.VMRuntime;
import libcore.io.IoUtils;
import libcore.io.Libcore;
import libcore.util.EmptyArray;
import static android.system.OsConstants._SC_NPROCESSORS_CONF;

/**
 * Every Java application has a single instance of class
 * <code>Runtime</code> that allows the application to interface with
 * the environment in which the application is running. The current
 * runtime can be obtained from the <code>getRuntime</code> method.
 * <p>
 * An application cannot create its own instance of this class.
 *
 * @author  unascribed
 * @see     java.lang.Runtime#getRuntime()
 * @since   JDK1.0
 */

public class Runtime {
    private static Runtime currentRuntime = new Runtime();

    /**
     * Holds the list of threads to run when the VM terminates
     */
    private List<Thread> shutdownHooks = new ArrayList<Thread>();

    /**
     * Reflects whether finalization should be run for all objects
     * when the VM terminates.
     */
    private static boolean finalizeOnExit;

    /**
     * Reflects whether we are already shutting down the VM.
     */
    private boolean shuttingDown;

    /**
     * Reflects whether we are tracing method calls.
     */
    private boolean tracingMethods;

    private static native void nativeExit(int code);

    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class <code>Runtime</code> are instance
     * methods and must be invoked with respect to the current runtime object.
     *
     * @return  the <code>Runtime</code> object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() {
        return currentRuntime;
    }

    /** Don't let anyone else instantiate this class */
    private Runtime() {}

    /**
     * Terminates the currently running Java virtual machine by initiating its
     * shutdown sequence.  This method never returns normally.  The argument
     * serves as a status code; by convention, a nonzero status code indicates
     * abnormal termination.
     *
     * <p> The virtual machine's shutdown sequence consists of two phases.  In
     * the first phase all registered {@link #addShutdownHook shutdown hooks},
     * if any, are started in some unspecified order and allowed to run
     * concurrently until they finish.  In the second phase all uninvoked
     * finalizers are run if {@link #runFinalizersOnExit finalization-on-exit}
     * has been enabled.  Once this is done the virtual machine {@link #halt
     * halts}.
     *
     * <p> If this method is invoked after the virtual machine has begun its
     * shutdown sequence then if shutdown hooks are being run this method will
     * block indefinitely.  If shutdown hooks have already been run and on-exit
     * finalization has been enabled then this method halts the virtual machine
     * with the given status code if the status is nonzero; otherwise, it
     * blocks indefinitely.
     *
     * <p> The <tt>{@link System#exit(int) System.exit}</tt> method is the
     * conventional and convenient means of invoking this method. <p>
     *
     * @param  status
     *         Termination status.  By convention, a nonzero status code
     *         indicates abnormal termination.
     *
     * @throws SecurityException
     *         If a security manager is present and its <tt>{@link
     *         SecurityManager#checkExit checkExit}</tt> method does not permit
     *         exiting with the specified status
     *
     * @see java.lang.SecurityException
     * @see java.lang.SecurityManager#checkExit(int)
     * @see #addShutdownHook
     * @see #removeShutdownHook
     * @see #runFinalizersOnExit
     * @see #halt(int)
     */
    public void exit(int status) {
        // Make sure we don't try this several times
        synchronized(this) {
            if (!shuttingDown) {
                shuttingDown = true;

                Thread[] hooks;
                synchronized (shutdownHooks) {
                    // create a copy of the hooks
                    hooks = new Thread[shutdownHooks.size()];
                    shutdownHooks.toArray(hooks);
                }

                // Start all shutdown hooks concurrently
                for (Thread hook : hooks) {
                    hook.start();
                }

                // Wait for all shutdown hooks to finish
                for (Thread hook : hooks) {
                    try {
                        hook.join();
                    } catch (InterruptedException ex) {
                        // Ignore, since we are at VM shutdown.
                    }
                }

                // Ensure finalization on exit, if requested
                if (finalizeOnExit) {
                    runFinalization();
                }

                // Get out of here finally...
                nativeExit(status);
            }
        }
    }

    /**
     * Registers a new virtual-machine shutdown hook.
     *
     * <p> The Java virtual machine <i>shuts down</i> in response to two kinds
     * of events:
     *
     *   <ul>
     *
     *   <p> <li> The program <i>exits</i> normally, when the last non-daemon
     *   thread exits or when the <tt>{@link #exit exit}</tt> (equivalently,
     *   <tt>{@link System#exit(int) System.exit}</tt>) method is invoked, or
     *
     *   <p> <li> The virtual machine is <i>terminated</i> in response to a
     *   user interrupt, such as typing <tt>^C</tt>, or a system-wide event,
     *   such as user logoff or system shutdown.
     *
     *   </ul>
     *
     * <p> A <i>shutdown hook</i> is simply an initialized but unstarted
     * thread.  When the virtual machine begins its shutdown sequence it will
     * start all registered shutdown hooks in some unspecified order and let
     * them run concurrently.  When all the hooks have finished it will then
     * run all uninvoked finalizers if finalization-on-exit has been enabled.
     * Finally, the virtual machine will halt.  Note that daemon threads will
     * continue to run during the shutdown sequence, as will non-daemon threads
     * if shutdown was initiated by invoking the <tt>{@link #exit exit}</tt>
     * method.
     *
     * <p> Once the shutdown sequence has begun it can be stopped only by
     * invoking the <tt>{@link #halt halt}</tt> method, which forcibly
     * terminates the virtual machine.
     *
     * <p> Once the shutdown sequence has begun it is impossible to register a
     * new shutdown hook or de-register a previously-registered hook.
     * Attempting either of these operations will cause an
     * <tt>{@link IllegalStateException}</tt> to be thrown.
     *
     * <p> Shutdown hooks run at a delicate time in the life cycle of a virtual
     * machine and should therefore be coded defensively.  They should, in
     * particular, be written to be thread-safe and to avoid deadlocks insofar
     * as possible.  They should also not rely blindly upon services that may
     * have registered their own shutdown hooks and therefore may themselves in
     * the process of shutting down.  Attempts to use other thread-based
     * services such as the AWT event-dispatch thread, for example, may lead to
     * deadlocks.
     *
     * <p> Shutdown hooks should also finish their work quickly.  When a
     * program invokes <tt>{@link #exit exit}</tt> the expectation is
     * that the virtual machine will promptly shut down and exit.  When the
     * virtual machine is terminated due to user logoff or system shutdown the
     * underlying operating system may only allow a fixed amount of time in
     * which to shut down and exit.  It is therefore inadvisable to attempt any
     * user interaction or to perform a long-running computation in a shutdown
     * hook.
     *
     * <p> Uncaught exceptions are handled in shutdown hooks just as in any
     * other thread, by invoking the <tt>{@link ThreadGroup#uncaughtException
     * uncaughtException}</tt> method of the thread's <tt>{@link
     * ThreadGroup}</tt> object.  The default implementation of this method
     * prints the exception's stack trace to <tt>{@link System#err}</tt> and
     * terminates the thread; it does not cause the virtual machine to exit or
     * halt.
     *
     * <p> In rare circumstances the virtual machine may <i>abort</i>, that is,
     * stop running without shutting down cleanly.  This occurs when the
     * virtual machine is terminated externally, for example with the
     * <tt>SIGKILL</tt> signal on Unix or the <tt>TerminateProcess</tt> call on
     * Microsoft Windows.  The virtual machine may also abort if a native
     * method goes awry by, for example, corrupting internal data structures or
     * attempting to access nonexistent memory.  If the virtual machine aborts
     * then no guarantee can be made about whether or not any shutdown hooks
     * will be run. <p>
     *
     * @param   hook
     *          An initialized but unstarted <tt>{@link Thread}</tt> object
     *
     * @throws  IllegalArgumentException
     *          If the specified hook has already been registered,
     *          or if it can be determined that the hook is already running or
     *          has already been run
     *
     * @throws  IllegalStateException
     *          If the virtual machine is already in the process
     *          of shutting down
     *
     * @throws  SecurityException
     *          If a security manager is present and it denies
     *          <tt>{@link RuntimePermission}("shutdownHooks")</tt>
     *
     * @see #removeShutdownHook
     * @see #halt(int)
     * @see #exit(int)
     * @since 1.3
     */
    public void addShutdownHook(Thread hook) {
        // Sanity checks
        if (hook == null) {
            throw new NullPointerException("hook == null");
        }

        if (shuttingDown) {
            throw new IllegalStateException("VM already shutting down");
        }

        if (hook.started) {
            throw new IllegalArgumentException("Hook has already been started");
        }

        synchronized (shutdownHooks) {
            if (shutdownHooks.contains(hook)) {
                throw new IllegalArgumentException("Hook already registered.");
            }

            shutdownHooks.add(hook);
        }
    }

    /**
     * De-registers a previously-registered virtual-machine shutdown hook. <p>
     *
     * @param hook the hook to remove
     * @return <tt>true</tt> if the specified hook had previously been
     * registered and was successfully de-registered, <tt>false</tt>
     * otherwise.
     *
     * @throws  IllegalStateException
     *          If the virtual machine is already in the process of shutting
     *          down
     *
     * @throws  SecurityException
     *          If a security manager is present and it denies
     *          <tt>{@link RuntimePermission}("shutdownHooks")</tt>
     *
     * @see #addShutdownHook
     * @see #exit(int)
     * @since 1.3
     */
    public boolean removeShutdownHook(Thread hook) {
        // Sanity checks
        if (hook == null) {
            throw new NullPointerException("hook == null");
        }

        if (shuttingDown) {
            throw new IllegalStateException("VM already shutting down");
        }

        synchronized (shutdownHooks) {
            return shutdownHooks.remove(hook);
        }
    }

    /**
     * Forcibly terminates the currently running Java virtual machine.  This
     * method never returns normally.
     *
     * <p> This method should be used with extreme caution.  Unlike the
     * <tt>{@link #exit exit}</tt> method, this method does not cause shutdown
     * hooks to be started and does not run uninvoked finalizers if
     * finalization-on-exit has been enabled.  If the shutdown sequence has
     * already been initiated then this method does not wait for any running
     * shutdown hooks or finalizers to finish their work. <p>
     *
     * @param  status
     *         Termination status.  By convention, a nonzero status code
     *         indicates abnormal termination.  If the <tt>{@link Runtime#exit
     *         exit}</tt> (equivalently, <tt>{@link System#exit(int)
     *         System.exit}</tt>) method has already been invoked then this
     *         status code will override the status code passed to that method.
     *
     * @throws SecurityException
     *         If a security manager is present and its <tt>{@link
     *         SecurityManager#checkExit checkExit}</tt> method does not permit
     *         an exit with the specified status
     *
     * @see #exit
     * @see #addShutdownHook
     * @see #removeShutdownHook
     * @since 1.3
     */
    public void halt(int status) {
        nativeExit(status);
    }

    /**
     * Enable or disable finalization on exit; doing so specifies that the
     * finalizers of all objects that have finalizers that have not yet been
     * automatically invoked are to be run before the Java runtime exits.
     * By default, finalization on exit is disabled.
     *
     * <p>If there is a security manager,
     * its <code>checkExit</code> method is first called
     * with 0 as its argument to ensure the exit is allowed.
     * This could result in a SecurityException.
     *
     * @param value true to enable finalization on exit, false to disable
     * @deprecated  This method is inherently unsafe.  It may result in
     *      finalizers being called on live objects while other threads are
     *      concurrently manipulating those objects, resulting in erratic
     *      behavior or deadlock.
     *
     * @throws  SecurityException
     *        if a security manager exists and its <code>checkExit</code>
     *        method doesn't allow the exit.
     *
     * @see     java.lang.Runtime#exit(int)
     * @see     java.lang.Runtime#gc()
     * @see     java.lang.SecurityManager#checkExit(int)
     * @since   JDK1.1
     */
    @Deprecated
    public static void runFinalizersOnExit(boolean value) {
        finalizeOnExit = value;
    }

    /**
     * Executes the specified string command in a separate process.
     *
     * <p>This is a convenience method.  An invocation of the form
     * <tt>exec(command)</tt>
     * behaves in exactly the same way as the invocation
     * <tt>{@link #exec(String, String[], File) exec}(command, null, null)</tt>.
     *
     * @param   command   a specified system command.
     *
     * @return  A new {@link Process} object for managing the subprocess
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@link SecurityManager#checkExec checkExec}
     *          method doesn't allow creation of the subprocess
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  NullPointerException
     *          If <code>command</code> is <code>null</code>
     *
     * @throws  IllegalArgumentException
     *          If <code>command</code> is empty
     *
     * @see     #exec(String[], String[], File)
     * @see     ProcessBuilder
     */
    public Process exec(String command) throws IOException {
        return exec(command, null, null);
    }

    /**
     * Executes the specified string command in a separate process with the
     * specified environment.
     *
     * <p>This is a convenience method.  An invocation of the form
     * <tt>exec(command, envp)</tt>
     * behaves in exactly the same way as the invocation
     * <tt>{@link #exec(String, String[], File) exec}(command, envp, null)</tt>.
     *
     * @param   command   a specified system command.
     *
     * @param   envp      array of strings, each element of which
     *                    has environment variable settings in the format
     *                    <i>name</i>=<i>value</i>, or
     *                    <tt>null</tt> if the subprocess should inherit
     *                    the environment of the current process.
     *
     * @return  A new {@link Process} object for managing the subprocess
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@link SecurityManager#checkExec checkExec}
     *          method doesn't allow creation of the subprocess
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  NullPointerException
     *          If <code>command</code> is <code>null</code>,
     *          or one of the elements of <code>envp</code> is <code>null</code>
     *
     * @throws  IllegalArgumentException
     *          If <code>command</code> is empty
     *
     * @see     #exec(String[], String[], File)
     * @see     ProcessBuilder
     */
    public Process exec(String command, String[] envp) throws IOException {
        return exec(command, envp, null);
    }

    /**
     * Executes the specified string command in a separate process with the
     * specified environment and working directory.
     *
     * <p>This is a convenience method.  An invocation of the form
     * <tt>exec(command, envp, dir)</tt>
     * behaves in exactly the same way as the invocation
     * <tt>{@link #exec(String[], String[], File) exec}(cmdarray, envp, dir)</tt>,
     * where <code>cmdarray</code> is an array of all the tokens in
     * <code>command</code>.
     *
     * <p>More precisely, the <code>command</code> string is broken
     * into tokens using a {@link StringTokenizer} created by the call
     * <code>new {@link StringTokenizer}(command)</code> with no
     * further modification of the character categories.  The tokens
     * produced by the tokenizer are then placed in the new string
     * array <code>cmdarray</code>, in the same order.
     *
     * @param   command   a specified system command.
     *
     * @param   envp      array of strings, each element of which
     *                    has environment variable settings in the format
     *                    <i>name</i>=<i>value</i>, or
     *                    <tt>null</tt> if the subprocess should inherit
     *                    the environment of the current process.
     *
     * @param   dir       the working directory of the subprocess, or
     *                    <tt>null</tt> if the subprocess should inherit
     *                    the working directory of the current process.
     *
     * @return  A new {@link Process} object for managing the subprocess
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@link SecurityManager#checkExec checkExec}
     *          method doesn't allow creation of the subprocess
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  NullPointerException
     *          If <code>command</code> is <code>null</code>,
     *          or one of the elements of <code>envp</code> is <code>null</code>
     *
     * @throws  IllegalArgumentException
     *          If <code>command</code> is empty
     *
     * @see     ProcessBuilder
     * @since 1.3
     */
    public Process exec(String command, String[] envp, File dir)
        throws IOException {
        if (command.length() == 0)
            throw new IllegalArgumentException("Empty command");

        StringTokenizer st = new StringTokenizer(command);
        String[] cmdarray = new String[st.countTokens()];
        for (int i = 0; st.hasMoreTokens(); i++)
            cmdarray[i] = st.nextToken();
        return exec(cmdarray, envp, dir);
    }

    /**
     * Executes the specified command and arguments in a separate process.
     *
     * <p>This is a convenience method.  An invocation of the form
     * <tt>exec(cmdarray)</tt>
     * behaves in exactly the same way as the invocation
     * <tt>{@link #exec(String[], String[], File) exec}(cmdarray, null, null)</tt>.
     *
     * @param   cmdarray  array containing the command to call and
     *                    its arguments.
     *
     * @return  A new {@link Process} object for managing the subprocess
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@link SecurityManager#checkExec checkExec}
     *          method doesn't allow creation of the subprocess
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  NullPointerException
     *          If <code>cmdarray</code> is <code>null</code>,
     *          or one of the elements of <code>cmdarray</code> is <code>null</code>
     *
     * @throws  IndexOutOfBoundsException
     *          If <code>cmdarray</code> is an empty array
     *          (has length <code>0</code>)
     *
     * @see     ProcessBuilder
     */
    public Process exec(String cmdarray[]) throws IOException {
        return exec(cmdarray, null, null);
    }

    /**
     * Executes the specified command and arguments in a separate process
     * with the specified environment.
     *
     * <p>This is a convenience method.  An invocation of the form
     * <tt>exec(cmdarray, envp)</tt>
     * behaves in exactly the same way as the invocation
     * <tt>{@link #exec(String[], String[], File) exec}(cmdarray, envp, null)</tt>.
     *
     * @param   cmdarray  array containing the command to call and
     *                    its arguments.
     *
     * @param   envp      array of strings, each element of which
     *                    has environment variable settings in the format
     *                    <i>name</i>=<i>value</i>, or
     *                    <tt>null</tt> if the subprocess should inherit
     *                    the environment of the current process.
     *
     * @return  A new {@link Process} object for managing the subprocess
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@link SecurityManager#checkExec checkExec}
     *          method doesn't allow creation of the subprocess
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  NullPointerException
     *          If <code>cmdarray</code> is <code>null</code>,
     *          or one of the elements of <code>cmdarray</code> is <code>null</code>,
     *          or one of the elements of <code>envp</code> is <code>null</code>
     *
     * @throws  IndexOutOfBoundsException
     *          If <code>cmdarray</code> is an empty array
     *          (has length <code>0</code>)
     *
     * @see     ProcessBuilder
     */
    public Process exec(String[] cmdarray, String[] envp) throws IOException {
        return exec(cmdarray, envp, null);
    }


    /**
     * Executes the specified command and arguments in a separate process with
     * the specified environment and working directory.
     *
     * <p>Given an array of strings <code>cmdarray</code>, representing the
     * tokens of a command line, and an array of strings <code>envp</code>,
     * representing "environment" variable settings, this method creates
     * a new process in which to execute the specified command.
     *
     * <p>This method checks that <code>cmdarray</code> is a valid operating
     * system command.  Which commands are valid is system-dependent,
     * but at the very least the command must be a non-empty list of
     * non-null strings.
     *
     * <p>If <tt>envp</tt> is <tt>null</tt>, the subprocess inherits the
     * environment settings of the current process.
     *
     * <p>A minimal set of system dependent environment variables may
     * be required to start a process on some operating systems.
     * As a result, the subprocess may inherit additional environment variable
     * settings beyond those in the specified environment.
     *
     * <p>{@link ProcessBuilder#start()} is now the preferred way to
     * start a process with a modified environment.
     *
     * <p>The working directory of the new subprocess is specified by <tt>dir</tt>.
     * If <tt>dir</tt> is <tt>null</tt>, the subprocess inherits the
     * current working directory of the current process.
     *
     * <p>If a security manager exists, its
     * {@link SecurityManager#checkExec checkExec}
     * method is invoked with the first component of the array
     * <code>cmdarray</code> as its argument. This may result in a
     * {@link SecurityException} being thrown.
     *
     * <p>Starting an operating system process is highly system-dependent.
     * Among the many things that can go wrong are:
     * <ul>
     * <li>The operating system program file was not found.
     * <li>Access to the program file was denied.
     * <li>The working directory does not exist.
     * </ul>
     *
     * <p>In such cases an exception will be thrown.  The exact nature
     * of the exception is system-dependent, but it will always be a
     * subclass of {@link IOException}.
     *
     *
     * @param   cmdarray  array containing the command to call and
     *                    its arguments.
     *
     * @param   envp      array of strings, each element of which
     *                    has environment variable settings in the format
     *                    <i>name</i>=<i>value</i>, or
     *                    <tt>null</tt> if the subprocess should inherit
     *                    the environment of the current process.
     *
     * @param   dir       the working directory of the subprocess, or
     *                    <tt>null</tt> if the subprocess should inherit
     *                    the working directory of the current process.
     *
     * @return  A new {@link Process} object for managing the subprocess
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          {@link SecurityManager#checkExec checkExec}
     *          method doesn't allow creation of the subprocess
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  NullPointerException
     *          If <code>cmdarray</code> is <code>null</code>,
     *          or one of the elements of <code>cmdarray</code> is <code>null</code>,
     *          or one of the elements of <code>envp</code> is <code>null</code>
     *
     * @throws  IndexOutOfBoundsException
     *          If <code>cmdarray</code> is an empty array
     *          (has length <code>0</code>)
     *
     * @see     ProcessBuilder
     * @since 1.3
     */
    public Process exec(String[] cmdarray, String[] envp, File dir)
        throws IOException {
        return new ProcessBuilder(cmdarray)
            .environment(envp)
            .directory(dir)
            .start();
    }

    /**
     * Returns the number of processors available to the Java virtual machine.
     *
     * <p> This value may change during a particular invocation of the virtual
     * machine.  Applications that are sensitive to the number of available
     * processors should therefore occasionally poll this property and adjust
     * their resource usage appropriately. </p>
     *
     * @return  the maximum number of processors available to the virtual
     *          machine; never smaller than one
     * @since 1.4
     */
    public int availableProcessors() {
        return (int) Libcore.os.sysconf(_SC_NPROCESSORS_CONF);
    }

    /**
     * Returns the amount of free memory in the Java Virtual Machine.
     * Calling the
     * <code>gc</code> method may result in increasing the value returned
     * by <code>freeMemory.</code>
     *
     * @return  an approximation to the total amount of memory currently
     *          available for future allocated objects, measured in bytes.
     */
    public native long freeMemory();

    /**
     * Returns the total amount of memory in the Java virtual machine.
     * The value returned by this method may vary over time, depending on
     * the host environment.
     * <p>
     * Note that the amount of memory required to hold an object of any
     * given type may be implementation-dependent.
     *
     * @return  the total amount of memory currently available for current
     *          and future objects, measured in bytes.
     */
    public native long totalMemory();

    /**
     * Returns the maximum amount of memory that the Java virtual machine will
     * attempt to use.  If there is no inherent limit then the value {@link
     * java.lang.Long#MAX_VALUE} will be returned. </p>
     *
     * @return  the maximum amount of memory that the virtual machine will
     *          attempt to use, measured in bytes
     * @since 1.4
     */
    public native long maxMemory();

    /**
     * Runs the garbage collector.
     * Calling this method suggests that the Java virtual machine expend
     * effort toward recycling unused objects in order to make the memory
     * they currently occupy available for quick reuse. When control
     * returns from the method call, the virtual machine has made
     * its best effort to recycle all discarded objects.
     * <p>
     * The name <code>gc</code> stands for "garbage
     * collector". The virtual machine performs this recycling
     * process automatically as needed, in a separate thread, even if the
     * <code>gc</code> method is not invoked explicitly.
     * <p>
     * The method {@link System#gc()} is the conventional and convenient
     * means of invoking this method.
     */
    public native void gc();

    /* Wormhole for calling java.lang.ref.Finalizer.runFinalization */
    private static native void runFinalization0();

    /**
     * Runs the finalization methods of any objects pending finalization.
     * Calling this method suggests that the Java virtual machine expend
     * effort toward running the <code>finalize</code> methods of objects
     * that have been found to be discarded but whose <code>finalize</code>
     * methods have not yet been run. When control returns from the
     * method call, the virtual machine has made a best effort to
     * complete all outstanding finalizations.
     * <p>
     * The virtual machine performs the finalization process
     * automatically as needed, in a separate thread, if the
     * <code>runFinalization</code> method is not invoked explicitly.
     * <p>
     * The method {@link System#runFinalization()} is the conventional
     * and convenient means of invoking this method.
     *
     * @see     java.lang.Object#finalize()
     */
    public void runFinalization() {
        VMRuntime.runFinalization(0);
    }

    /**
     * Enables/Disables tracing of instructions.
     * If the <code>boolean</code> argument is <code>true</code>, this
     * method suggests that the Java virtual machine emit debugging
     * information for each instruction in the virtual machine as it
     * is executed. The format of this information, and the file or other
     * output stream to which it is emitted, depends on the host environment.
     * The virtual machine may ignore this request if it does not support
     * this feature. The destination of the trace output is system
     * dependent.
     * <p>
     * If the <code>boolean</code> argument is <code>false</code>, this
     * method causes the virtual machine to stop performing the
     * detailed instruction trace it is performing.
     *
     * @param enable   <code>true</code> to enable instruction tracing;
     *               <code>false</code> to disable this feature.
     */
    // Android changed - param name s/on/enable
    public void traceInstructions(boolean enable) {
    }

    /**
     * Enables/Disables tracing of method calls.
     * If the <code>boolean</code> argument is <code>true</code>, this
     * method suggests that the Java virtual machine emit debugging
     * information for each method in the virtual machine as it is
     * called. The format of this information, and the file or other output
     * stream to which it is emitted, depends on the host environment. The
     * virtual machine may ignore this request if it does not support
     * this feature.
     * <p>
     * Calling this method with argument false suggests that the
     * virtual machine cease emitting per-call debugging information.
     *
     * @param enable   <code>true</code> to enable instruction tracing;
     *               <code>false</code> to disable this feature.
     */
    // Android changed - param name s/on/enable
    public void traceMethodCalls(boolean enable) {
        if (enable != tracingMethods) {
            if (enable) {
                VMDebug.startMethodTracing();
            } else {
                VMDebug.stopMethodTracing();
            }
            tracingMethods = enable;
        }
    }

    /**
     * Loads the specified filename as a dynamic library. The filename
     * argument must be a complete path name,
     * (for example
     * <code>Runtime.getRuntime().load("/home/avh/lib/libX11.so");</code>).
     * <p>
     * First, if there is a security manager, its <code>checkLink</code>
     * method is called with the <code>filename</code> as its argument.
     * This may result in a security exception.
     * <p>
     * This is similar to the method {@link #loadLibrary(String)}, but it
     * accepts a general file name as an argument rather than just a library
     * name, allowing any file of native code to be loaded.
     * <p>
     * The method {@link System#load(String)} is the conventional and
     * convenient means of invoking this method.
     *
     * @param      filename   the file to load.
     * @exception  SecurityException  if a security manager exists and its
     *             <code>checkLink</code> method doesn't allow
     *             loading of the specified dynamic library
     * @exception  UnsatisfiedLinkError  if the file does not exist.
     * @exception  NullPointerException if <code>filename</code> is
     *             <code>null</code>
     * @see        java.lang.Runtime#getRuntime()
     * @see        java.lang.SecurityException
     * @see        java.lang.SecurityManager#checkLink(java.lang.String)
     */
    @CallerSensitive
    public void load(String filename) {
        load0(VMStack.getStackClass1(), filename);
    }

    /** Check target sdk, if it's higher than N, we throw an UnsupportedOperationException */
    private void checkTargetSdkVersionForLoad(String methodName) {
        final int targetSdkVersion = VMRuntime.getRuntime().getTargetSdkVersion();
        if (targetSdkVersion > 24) {
            throw new UnsupportedOperationException(methodName + " is not supported on SDK " +
                                                    targetSdkVersion);
        }
    }

    // Fixes b/25859957 regression. Depending on private methods is bad, mkay.
    void load(String absolutePath, ClassLoader loader) {
        checkTargetSdkVersionForLoad("java.lang.Runtime#load(String, ClassLoader)");

        java.lang.System.logE("java.lang.Runtime#load(String, ClassLoader)" +
                              " is private and will be removed in a future Android release");
        if (absolutePath == null) {
            throw new NullPointerException("absolutePath == null");
        }
        String error = doLoad(absolutePath, loader);
        if (error != null) {
            throw new UnsatisfiedLinkError(error);
        }
    }

    synchronized void load0(Class fromClass, String filename) {
        if (!(new File(filename).isAbsolute())) {
            throw new UnsatisfiedLinkError(
                "Expecting an absolute path of the library: " + filename);
        }
        if (filename == null) {
            throw new NullPointerException("filename == null");
        }
        String error = doLoad(filename, fromClass.getClassLoader());
        if (error != null) {
            throw new UnsatisfiedLinkError(error);
        }
    }

    /**
     * Loads the dynamic library with the specified library name.
     * A file containing native code is loaded from the local file system
     * from a place where library files are conventionally obtained. The
     * details of this process are implementation-dependent. The
     * mapping from a library name to a specific filename is done in a
     * system-specific manner.
     * <p>
     * First, if there is a security manager, its <code>checkLink</code>
     * method is called with the <code>libname</code> as its argument.
     * This may result in a security exception.
     * <p>
     * The method {@link System#loadLibrary(String)} is the conventional
     * and convenient means of invoking this method. If native
     * methods are to be used in the implementation of a class, a standard
     * strategy is to put the native code in a library file (call it
     * <code>LibFile</code>) and then to put a static initializer:
     * <blockquote><pre>
     * static { System.loadLibrary("LibFile"); }
     * </pre></blockquote>
     * within the class declaration. When the class is loaded and
     * initialized, the necessary native code implementation for the native
     * methods will then be loaded as well.
     * <p>
     * If this method is called more than once with the same library
     * name, the second and subsequent calls are ignored.
     *
     * @param      libname   the name of the library.
     * @exception  SecurityException  if a security manager exists and its
     *             <code>checkLink</code> method doesn't allow
     *             loading of the specified dynamic library
     * @exception  UnsatisfiedLinkError  if the library does not exist.
     * @exception  NullPointerException if <code>libname</code> is
     *             <code>null</code>
     * @see        java.lang.SecurityException
     * @see        java.lang.SecurityManager#checkLink(java.lang.String)
     */
    @CallerSensitive
    public void loadLibrary(String libname) {
        loadLibrary0(VMStack.getCallingClassLoader(), libname);
    }

    /**
     * Temporarily preserved for backward compatibility. Applications call this
     * method using reflection.
     *
     * **** THIS METHOD WILL BE REMOVED IN A FUTURE ANDROID VERSION ****
     *
     * http://b/26217329
     *
     * @hide
     */
    public void loadLibrary(String libname, ClassLoader classLoader) {
        checkTargetSdkVersionForLoad("java.lang.Runtime#loadLibrary(String, ClassLoader)");
        java.lang.System.logE("java.lang.Runtime#loadLibrary(String, ClassLoader)" +
                              " is private and will be removed in a future Android release");
        loadLibrary0(classLoader, libname);
    }

    synchronized void loadLibrary0(ClassLoader loader, String libname) {
        if (libname.indexOf((int)File.separatorChar) != -1) {
            throw new UnsatisfiedLinkError(
    "Directory separator should not appear in library name: " + libname);
        }
        String libraryName = libname;
        if (loader != null) {
            String filename = loader.findLibrary(libraryName);
            if (filename == null) {
                // It's not necessarily true that the ClassLoader used
                // System.mapLibraryName, but the default setup does, and it's
                // misleading to say we didn't find "libMyLibrary.so" when we
                // actually searched for "liblibMyLibrary.so.so".
                throw new UnsatisfiedLinkError(loader + " couldn't find \"" +
                                               System.mapLibraryName(libraryName) + "\"");
            }
            String error = doLoad(filename, loader);
            if (error != null) {
                throw new UnsatisfiedLinkError(error);
            }
            return;
        }

        String filename = System.mapLibraryName(libraryName);
        List<String> candidates = new ArrayList<String>();
        String lastError = null;
        for (String directory : getLibPaths()) {
            String candidate = directory + filename;
            candidates.add(candidate);

            if (IoUtils.canOpenReadOnly(candidate)) {
                String error = doLoad(candidate, loader);
                if (error == null) {
                    return; // We successfully loaded the library. Job done.
                }
                lastError = error;
            }
        }

        if (lastError != null) {
            throw new UnsatisfiedLinkError(lastError);
        }
        throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
    }

    private volatile String[] mLibPaths = null;

    private String[] getLibPaths() {
        if (mLibPaths == null) {
            synchronized(this) {
                if (mLibPaths == null) {
                    mLibPaths = initLibPaths();
                }
            }
        }
        return mLibPaths;
    }

    private static String[] initLibPaths() {
        String javaLibraryPath = System.getProperty("java.library.path");
        if (javaLibraryPath == null) {
            return EmptyArray.STRING;
        }
        String[] paths = javaLibraryPath.split(":");
        // Add a '/' to the end of each directory so we don't have to do it every time.
        for (int i = 0; i < paths.length; ++i) {
            if (!paths[i].endsWith("/")) {
                paths[i] += "/";
            }
        }
        return paths;
    }
    private String doLoad(String name, ClassLoader loader) {
        // Android apps are forked from the zygote, so they can't have a custom LD_LIBRARY_PATH,
        // which means that by default an app's shared library directory isn't on LD_LIBRARY_PATH.

        // The PathClassLoader set up by frameworks/base knows the appropriate path, so we can load
        // libraries with no dependencies just fine, but an app that has multiple libraries that
        // depend on each other needed to load them in most-dependent-first order.

        // We added API to Android's dynamic linker so we can update the library path used for
        // the currently-running process. We pull the desired path out of the ClassLoader here
        // and pass it to nativeLoad so that it can call the private dynamic linker API.

        // We didn't just change frameworks/base to update the LD_LIBRARY_PATH once at the
        // beginning because multiple apks can run in the same process and third party code can
        // use its own BaseDexClassLoader.

        // We didn't just add a dlopen_with_custom_LD_LIBRARY_PATH call because we wanted any
        // dlopen(3) calls made from a .so's JNI_OnLoad to work too.

        // So, find out what the native library search path is for the ClassLoader in question...
        String librarySearchPath = null;
        if (loader != null && loader instanceof BaseDexClassLoader) {
            BaseDexClassLoader dexClassLoader = (BaseDexClassLoader) loader;
            librarySearchPath = dexClassLoader.getLdLibraryPath();
        }
        // nativeLoad should be synchronized so there's only one LD_LIBRARY_PATH in use regardless
        // of how many ClassLoaders are in the system, but dalvik doesn't support synchronized
        // internal natives.
        synchronized (this) {
            return nativeLoad(name, loader, librarySearchPath);
        }
    }

    // TODO: should be synchronized, but dalvik doesn't support synchronized internal natives.
    private static native String nativeLoad(String filename, ClassLoader loader,
                                            String librarySearchPath);

    /**
     * Creates a localized version of an input stream. This method takes
     * an <code>InputStream</code> and returns an <code>InputStream</code>
     * equivalent to the argument in all respects except that it is
     * localized: as characters in the local character set are read from
     * the stream, they are automatically converted from the local
     * character set to Unicode.
     * <p>
     * If the argument is already a localized stream, it may be returned
     * as the result.
     *
     * @param      in InputStream to localize
     * @return     a localized input stream
     * @see        java.io.InputStream
     * @see        java.io.BufferedReader#BufferedReader(java.io.Reader)
     * @see        java.io.InputStreamReader#InputStreamReader(java.io.InputStream)
     * @deprecated As of JDK&nbsp;1.1, the preferred way to translate a byte
     * stream in the local encoding into a character stream in Unicode is via
     * the <code>InputStreamReader</code> and <code>BufferedReader</code>
     * classes.
     */
    @Deprecated
    public InputStream getLocalizedInputStream(InputStream in) {
        return in;
    }

    /**
     * Creates a localized version of an output stream. This method
     * takes an <code>OutputStream</code> and returns an
     * <code>OutputStream</code> equivalent to the argument in all respects
     * except that it is localized: as Unicode characters are written to
     * the stream, they are automatically converted to the local
     * character set.
     * <p>
     * If the argument is already a localized stream, it may be returned
     * as the result.
     *
     * @deprecated As of JDK&nbsp;1.1, the preferred way to translate a
     * Unicode character stream into a byte stream in the local encoding is via
     * the <code>OutputStreamWriter</code>, <code>BufferedWriter</code>, and
     * <code>PrintWriter</code> classes.
     *
     * @param      out OutputStream to localize
     * @return     a localized output stream
     * @see        java.io.OutputStream
     * @see        java.io.BufferedWriter#BufferedWriter(java.io.Writer)
     * @see        java.io.OutputStreamWriter#OutputStreamWriter(java.io.OutputStream)
     * @see        java.io.PrintWriter#PrintWriter(java.io.OutputStream)
     */
    @Deprecated
    public OutputStream getLocalizedOutputStream(OutputStream out) {
        return out;
    }

}